In methods for obtaining images by electrophotographic printing and electrostatic development, electrostatic charge images formed on photosensitive materials are fixed after being developed by toner that has been electrified by friction. Fixing systems include heat roller systems that fix developed toner images using pressurized heated rollers, and non-contact fixing systems whereby fixing is accomplished using electric ovens or flash beam light.
For these processes to be performed smoothly, the toner must firstly be able to hold a stable electrostatic charge, and secondly must satisfactorily fix onto paper. Also, since the apparatus has a heating element in the fusing section that causes the temperature in the apparatus to increase, the toner must be one that does not undergo blocking in the apparatus.
The increasing emphasis on reducing energy consumption in recent years has spurred advances in the area of reducing temperatures in the fusing sections in heat roller systems. This has resulted in greater demand for toner with the ability to affix to paper at low temperature, also known as low temperature fixability. In addition, offset resistance is considered a standard requirement in consideration of the offset phenomenon that occurs in heat roller systems. It is therefore necessary for toner to exhibit low-temperature fixability whereby it becomes fixed to paper under conditions with a fixing temperature of 140° C., for example, while maintaining offset resistance, while toner is also desired which has a wider working range, such as a fixing temperature range of 50° C. or above.
Toner binding resins have a major effect on the aforementioned toner properties and include polystyrene resins, styrene-acrylic resins, polyester resins, epoxy resins, polyamide resins and the like, among which polyester resins have attracted particular interest recently in light of more easily obtaining balance between transparency and fixability.
It has been attempted in the past to broaden the fixing temperature range of polyester resins by using non-linear polyester resins having three-dimensional crosslinked structures comprising trifunctional or greater monomers (for example, see Patent document 1). However, while the non-linear polyester resins disclosed in Patent document 1 have excellent high-temperature offset resistance and exhibit a high maximum fixing temperature, the level of low-temperature fixability has not been satisfactory.
Means for improving the low-temperature fixability have been investigated using linear polyester resins composed of divalent carboxylic acid compounds and divalent alcohol compounds (for example, see Patent document 2). However, linear polyester resins without a three-dimensional structure exhibit poor high-temperature offset resistance despite their excellent low-temperature fixability, and as a result the fixing temperature range is not very broad.
It has also been attempted to introduce unsaturated groups into linear polyester resins for reaction and crosslinking with polymerization initiators (for example, see Patent documents 3-6).
Patent document 3, for example, describes a technique involving crosslinking reaction of an unsaturated polyester resin composed of a divalent carboxylic acid such as isophthalic acid or maleic anhydride and a divalent alcohol such as bisphenol A propylene oxide addition product, using dicumyl peroxide as the polymerization initiator. Toner employing crosslinked polyester resins obtained in such a manner are characterized by favorable fixability and absence of the “marking back” phenomenon, but the large amount of organic peroxide used, about 0.5-20 mass % with respect to the resin, results in significant residue of decomposition products in the resin and a lower storage stability.
Patent document 4 describes crosslinking reaction of an unsaturated polyester resin composed of fumaric acid with bisphenol A propylene oxide addition product or the like to obtain a toner resin that includes a crosslinked fraction and non-crosslinked fraction. The toner described in Patent document 4, however, has a major drawback of low durability.
Patent document 5 describes a toner resin obtained by heating an unsaturated polyester resin having a number-average molecular weight of 1,000-4,000 and an unsaturated double bond, or further curing it in the presence of a polymerization initiator. The toner described in Patent document 5, however, has an unsatisfactory fixing range.
The toner described in Patent document 6 is composed of the product of crosslinking an unsaturated polyester resin and a saturated polyester resin with a melt start temperature of between 50° C. and 100° C. The toner has a wide non-offset range but insufficient image fixing strength at low temperatures of 140° C. and below.
Therefore, no polyester resin for toner exists that can produce a toner with excellent high-temperature offset resistance together with a wide fixing range of 50° C. and above, sufficient fixing strength even at fixing temperatures of 140° C. and below, and satisfactory storage stability and durability.
Patent document 1: Japanese Unexamined Patent Publication SHO No. 57-109825
Patent document 2: Japanese Unexamined Patent Publication HEI No. 4-12367
Patent document 3: Japanese Unexamined Patent Publication HEI No. 3-135578
Patent document 4: Japanese Unexamined Patent Publication HEI No. 6-130722
Patent document 5: Japanese Unexamined Patent Publication SHO No. 59-49551
Patent document 6: Japanese Unexamined Patent Publication HEI No. 8-152743